1. Field of the Invention
The invention relates to a device for reducing the axial force load of a fluid supply pump, such as a gear pump, which is used as a fuel delivery pump to supply fuel to the internal combustion engine from the tank of a motor vehicle.
2. Description of the Prior Art
DE 195 138 22 A1 has disclosed a device for delivering fuel from a tank to an internal combustion engine used to drive a motor vehicle. In this embodiment, a delivery line from the tank to the engine contains a feed pump which has at least one pump chamber in its housing. The pump chamber contains a pair of meshed rotary driven gears, which feed fuel from an intake chamber that can be connected to the tank, along a supply conduit embodied between the face of the gears and the wall, into a pressure chamber that can be connected at least indirectly to the internal combustion engine. In order to reduce the structural space and the manufacturing costs, the housing of the feed pump contains an additional air supply pump whose suction side is connected to a power brake booster of the motor vehicle.
In gear pumps used to supply fuel from motor vehicle tanks, pressures of up to 8 bar can occur. The exertion of up to 8 bar of pressure, which is directed essentially from the delivery side toward the suction side of the pump, produces an axial force that acts on the delivery side feed elements of the pump, which presses an end face of the shaft against a seat face. After long-term operation, signs of wear appear on the seat face; this results in premature wear on the end face of the shaft and on the seat face. One attempt to prevent premature wear on an end face of the shaft was comprised of providing the boundary surface at the intake end of the shaft with a relief groove, which connected the end face of the shaft opposite from the boundary surface to the suction side of the delivery device. But this involves an additional operating phase of milling out the relief groove during production of the boundary surface.
With the configuration according to the invention of a fuel supply pump, on the one hand its manufacture can be simplified and it can therefore be embodied in a less expensive manner. The wall which defines the feed device on the suction side can now be produced without the additional operating phase of milling out a relief groove in the region opposite from the end face of the shaft contained in the housing. A surface treatment of the groove in the limit wall provided on the suction side in order to increase the surface hardness can also be eliminated so that as a whole, this component is easier to produce. On the other hand, the embodiment according to the invention permits a reduction of the axial force acting on one side of the shaft contained in the housing. Up till now, the axial force produced has resulted in premature wear on an end face due to frictional contact with the seat face of the shaft. By means of the conduit system in the housing of the feed device and in the shaft itself, a relief force, which acts on the shaft in opposition to the axial force, can be produced on a surface area enlargement on the end face of the shaft disposed opposite from the boundary surface. The relief force counteracts the axial force in the shaft and is produced by the application of a negative pressure to a surface area enlargement in the form of a concave curved recess on the end face of the shaft.
In another embodiment of the invention, the conduit system extends in the shaft, underneath a sealing ring supported on the shaft, which ring, on the end face, seals a hollow chamber embodied as a circumferential groove. This hollow chamber communicates with the suction side of the feed device by means of a conduit in the housing.
By means of the conduit system, the negative pressure is applied from the suction side of the feed device to the end face of the shaft where the surface area enlargement is embodied.
The surface area enlargement embodied on the end face of the shaft, for example in the form of a concave curved recess, permits an enlarged effective area for the application of the negative pressure. The greater the surface area enlargement can be embodied, the greater the relief pressure acting on the shaft can be. As a result, the axial force on the shaft generated by the impingement of pressure on the feed device can be counteracted so that premature frictional wear between the limit wall and the end face of the shaft disposed opposite it can be counteracted.
Consequently, an adjustable force equilibrium on the shaft keeps the end face of the shaft from contacting the opposing limit wall and prevents premature wear.